Technical Field
The present invention relates to photovoltaic devices, and more particularly to formation methods and devices using absorber layers with a graded bandgap profile.
Description of the Related Art
Cu—In—Ga—S/Se (CIGSSe) technology provides high performance solar cells with very high power conversion efficiency (PCE) (e.g., about 20%). CIGSSe solar cells have a very large open circuit voltage (V∝) relative to bandgap with no known issues of interface recombination. Unfortunately, the reliance on rare elements, such as indium, for example, limits very large scale deployment of this technology.
Cu—Zn—Sn—S/Se (CZTSSe) is an emerging thin film solar cell technology consisting of all earth abundant elements. While progress has been made in the development of CZTSSe solar cells particularly using hydrazine-based solution processing, a PCE of only about 12.6% has been achieved.
Several major limitations in CZTSSe solar cells exist as well. For example, a low V∝ may be experienced, which is suspected to be due to high buffer-absorber interface recombination, high bulk defect states, existence of tail states in the bulk and possible Fermi level pinning in the bulk or at an interface. Furthermore, CZTSSe also suffers from low fill factor (FF) which is mostly due to low V∝ and higher series resistance from various layers or potential barrier formation across the device.